A variety of optical communications modules exist for transmitting and/or receiving optical data signals over optical waveguides (e.g., optical fibers). Optical communications modules include optical receiver modules, optical transmitter modules and optical transceiver modules. Optical receiver modules have one or more receive channels for receiving one or more optical data signals over one or more respective optical waveguides. Optical transmitter modules have one or more transmit channels for transmitting one or more optical data signals over one or more respective optical waveguides. Optical transceiver modules have one or more transmit channels and one or more receive channels for transmitting and receiving respective optical transmit and receive data signals over respective transmit and receive optical waveguides.
For each of these different types of optical communications modules, a variety of designs and configurations exist. A typical layout of an optical communications module includes a module circuit board, such as a printed circuit board (PCB), a ball grid array (BGA), or the like, and various electrical components and optoelectronic components mounted on the module circuit board. In the case of an optical transmitter module, one or more light sources such as laser diodes or light-emitting diodes (LEDs) and one or more driver integrated circuits (ICs) are mounted on the module circuit board and electrically interconnected with it.
Similar configurations are used for optical receiver modules, except that the module circuit board has one or more light detectors instead of light sources mounted on it and has a receiver IC instead of a driver IC mounted on it. Optical transceiver modules typically have one or more light sources, one or more light detectors, a light source driver IC, and receiver IC mounted on the module circuit board.
Optical communications modules of the type described above are sometimes mounted directly on the module PCB, but sometimes the modules are mounted on a separate secondary PCB sometimes called a “daughter card.” These secondary PCBs are sometimes incorporated into the packaging of other electronics used in the module, such as in the packaging of an application specific integrated circuit (ASIC). These secondary PCBs can be made of conventional PCB material or of a specialized, high performance material called “organic substrate.”
The PCB often has a controller IC mounted on it that is in communication with the driver and/or receiver ICs through electrical conductors (traces and/or vias) of the PCB and electrical conductors (traces and/or vias) of the module circuit board in a design that has multiple PCBs. Bond wires are typically also used to make some of the interconnections between ICs and the PCB on which the ICs are mounted. One problem that can occur with this type of configuration is that the electrically-conductive pathways between components mounted on the PCB and components mounted on the sub-circuit boards are often so long that electrical errors can be introduced into the signal. Errors can include inductive or capacitive coupling between adjacent electrically-conductive pathways, or signal filtering that results from the inductance, capacitance and resistance of such paths. Such coupling or filtering can degrade signal integrity and overall performance, especially at higher data rates.
The PCB and the optical communications module mounted thereon are typically housed in a metal housing that is configured to be plugged, or inserted, into a metal cage. The metal cage is often held in a slot of a rack that has multiple slots for holding multiple cages. The PCB and sub-modules typically include one or more heat sink devices for dissipating heat generated by the electrical and optoelectronic components mounted thereon. The heat sink devices are typically thermal pads, lead frames or the metal housing on which the electrical and optoelectronic components are mounted. The electrical and optoelectronic components are typically attached to the heat sink devices by a thermally conductive material to enable heat generated by them to pass into and dissipate in the heat sink devices. The heat sink devices spread out the heat to move it away from the components.
Heat generated by the electrical and optoelectronic components can detrimentally affect the performance of the optical communications module. External heat dissipation structures are often mechanically coupled to the metal housings of the optical communications modules to allow heat generated by the electrical and optoelectronic components to be transferred from the module housing into the external heat dissipation structure. This helps lower the temperature inside of the module housing.
Because of an ever-increasing need to increase the bandwidth of optical links, efforts are constantly being made to increase the operating speeds or data rates of the laser diodes used in the modules. As the speeds of laser diodes are increased to achieve higher link data rates, their temperatures must be reduced. Increases in link length also require reductions in the temperatures of the laser diodes. In order to meet these needs, heat dissipation solutions should be highly effective at dissipating heat. If they are not, then the temperature of the laser diode may increase to the point that its performance is detrimentally affected. The heat dissipation solutions also need to be cost effective. Providing heat dissipation solutions for high-speed optical communications modules that are cost effective and effective at dissipating heat continues to be a challenge in the industry.
Another issue that needs to be addressed when designing optical communications modules is EMI shielding. In most optical communications modules, the receptacle that receives the optical connector disposed on the end of the optical fiber cable constitutes an EMI open aperture that allows EMI to escape from the module housing. The Federal Communications Commission (FCC) has set standards that limit the amount of electromagnetic radiation that may emanate from unintended sources. For this reason, a variety of techniques and designs are used to shield EMI open apertures in module housings in order to limit the amount of EMI that passes through the apertures. Various metal shielding designs and resins that contain metallic material have been used to cover areas from which EMI may escape from the housings. So far, such techniques and designs have had only limited success, especially with respect to optical communications modules that transmit and/or receive data at very high data rates (e.g., 10 gigabits per second (Gbps) and higher).
For example, EMI collars are often used with pluggable optical communications modules to provide EMI shielding. The EMI collars in use today vary in construction, but generally include a band portion that is secured about the exterior of the module housing and spring fingers having proximal ends that attach to the band portion and distal ends that extend away from the proximal ends. The spring fingers are periodically spaced about the collar and have folds in them near their distal ends that direct the distal ends inwardly toward the module housing. The distal ends make contact with the housing at periodically-spaced points on the housing. At the locations where the folds occur near the distal ends of the spring fingers, the outer surfaces of the spring fingers are in contact with the inner surface of the cage at periodically spaced contact points along the inner surface of the cage. Such EMI collar designs are based on Faraday cage principles.
The amount of EMI that passes through an EMI shielding device is proportional to the largest dimension of the largest EMI open aperture of the EMI shielding device. Therefore, EMI shielding devices such as EMI collars and other devices are designed to ensure that there is no open aperture that has a dimension that exceeds the maximum allowable EMI open aperture dimension associated with the frequency of interest. For example, in the known EMI collars of the type described above, the spacing between the locations at which the distal ends of the spring fingers come into contact with the inner surface of the cage should not exceed one quarter wavelength of the frequency of interest that is being attenuated. Even greater attenuation of the frequency of interest can be achieved by making the maximum EMI open aperture dimension significantly less than one quarter of a wavelength, such as, for example, one eighth or one tenth of a wavelength. However, the ability to decrease this spacing using currently available manufacturing techniques is limited. In addition, as the frequencies of optical communications modules increase, this spacing needs to be made smaller in order to effectively shield EMI, which becomes increasingly difficult or impossible to achieve at very high frequencies.
Accordingly, a need exists for an optical communications module configuration and method that provide improvements in heat dissipation, EMI shielding and signal integrity.